Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for deghosting wave-fields collected with receivers located either on streamers or on or close to the ocean bottom.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 110 tows plural detectors 112, which are disposed along a cable 114. Cable 114 together with its corresponding detectors 112 are sometimes referred to, by those skilled in the art, as a streamer 116. Vessel 110 may tow plural streamers 116 at the same time. Streamers may be disposed horizontally, i.e., lie at a constant depth z1 relative to the ocean surface 118. Also, plural streamers 116 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. In one embodiment, the streamers may have a curved profile as described, for example, in U.S. Pat. No. 8,593,904, the entire content of which is incorporated herein by reference.
Still with reference to FIG. 1, vessel 110 may also tow a seismic source 120 configured to generate an acoustic wave 122a. Acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector R). Reflected acoustic wave 122b propagates upward and is detected by detector 112. For simplicity, FIG. 1 shows only two paths 122a corresponding to the acoustic wave. Parts of reflected acoustic wave 122b (primary) are recorded by various detectors 112 (recorded signals are called traces) while parts of reflected wave 122c pass detectors 112 and arrive at the water surface 118. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for acoustic waves), reflected wave 122c is reflected back toward detector 112 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave because it is due to a spurious reflection. Ghosts are also recorded by detector 112, but with a reverse polarity and a time lag relative to primary wave 122b if the detector is a hydrophone. The degenerative effect that ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by detectors.
Recorded traces may be used to determine the subsurface (i.e., earth structure below surface 124) and to determine the position and presence of reflectors 126, which is associated with the detection of oil and/or gas reservoirs. However, ghosts disturb the accuracy of the final image of the subsurface and, for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the acquired seismic data.
Research studies have been performed to use the pressure data (P) and the particle velocity (Vz) to remove the receiver ghost (e.g., U.S. Pat. Nos. 4,437,175, 4,486,865). Different from the pressure data recorded by hydrophones, particle velocity is measured by geophones or accelerometers that bear the vertical direction (up or down) of the wave propagation. The up-going wave-fields (the primary) detected by the geophone and hydrophone are in-phase, and the down-going wave-fields (the receiver ghost) are 180° out-of-phase. Therefore, these two components are complementary to each other in terms of receiver ghost attenuation.
For variable-depth streamers, a deghosting process has been disclosed, for example, in U.S. Pat. No. 8,456,951 (herein '951) authored by R Soubaras, the entire content of which is incorporated herein. According to the '951 patent, a method for deghosting uses joint deconvolution for migration and mirror migration images to generate a final image of a subsurface. Deghosting is performed at the end of processing (during an imaging phase) and not at the beginning, as with traditional methods. Further, the '951 patent discloses that no datuming step is performed on the data.
Another method that addresses variable-depth data is disclosed by U.S. patent application Ser. No. 13/334,776 (herein 776) authored by G. Poole. This method uses a surface datum τ-p model that represents input shot data. A transform from the τ-p model to a shot domain (offset-time) combines the operations of redatuming and reghosting. The use of variable-depth streamer data combined with reghosting ensures that a single point in the τ-p domain satisfies a range of different ghost lags, therefore, making use of variable-depth data notch diversity, which ensures effective receiver deghosting.
However, existing methods relate to pressure measurements made, for example, by hydrophones. Currently, the new streamer generation is capable of measuring not only pressure but also particle motion data, e.g., displacement, velocity, differential pressure, acceleration, etc. Thus, there is a need to process not only pressure measurements, but also particle motion data. Accordingly, it would be desirable to provide systems and methods with such capabilities.